This invention relates to air conditioning and heat pump climate control systems and, particularly, to a method of operation control using a thermostat equipped with a humidity sensor to control the humidity in an enclosed space.
In the design of an air conditioner or a heat pump climate control system, it is general practice to use an indoor blower equipped to operate at one of two or three possible speeds. As such, system performance and efficiency is optimized at a few sets of a much larger number of possible sets of operating conditions. System performance at other conditions or blower speeds can be less than optimal but is considered acceptable on the basis of system design economy. System control is accomplished using a thermostat to cycle the compressor and blower on and off in response to a demand for sensible cooling, thereby, maintaining the temperature inside the structure at a desired level.
The humidity control capability of an air conditioning system is expressed as the ratio of sensible heat capacity to the total heat capacity and referred to as the Sensible Heat Ratio (SHR). The SHR is a function of the evaporator operating temperature, evaporator surface area, the dew point and the amount of the air entering the evaporator. One problem with typical system design is that the humidity control capability of the system is, in general, not optimized to the latent load requirements of the structure during all of its seasonal weather conditions. That is to say, while the air conditioner or heat pump is in satisfactory control of the sensible structure load, the humidity level is most often out of control. In fact, the indoor humidity levels during sensible load conditions considered "light loading" greatly exceed levels generally considered comfortable.
At outdoor dry bulb temperatures in the 65-85.degree. F. range, air conditioning systems experience long idle times. That is, when the outdoor temperature is near or below the indoor temperature setpoint, the air conditioner is off and no cooling or dehumidification is performed. As at these outdoor temperatures, there is almost always a need for dehumidification the indoor humidity may rise to unacceptable levels resulting in mold, mildew and dust mite growth conditions. In addition, without dehumidification, the moisture content in the building structural materials rises which may cause damage. If the humidity is allowed to raise excessively, condensation could form on interior surfaces and could cause furniture damage. The problem is therefore, to develop a cooling system control that provides for dehumidification operation during idle periods which will keep humidity levels low enough to discourage or eliminate the growth of mold, mildew and dust mites and to prevent damage to the building and furniture.
Many homes and structures remain unoccupied by humans for extended periods of time. These long unoccupied periods pose a special problem in terms of dehumidification. Leaving the HVAC system to operate in order to dehumidify the space while unoccupied needlessly over cools the space and is energy inefficient. If the HVAC system is not operated the space will encounter long periods when moisture will accumulate and will promote the growth of mold and mildew. In addition, if the HVAC system is not operated for long periods and moisture is permitted to accumulate the HVAC system, once restarted, will require an extended period to dehumidify the space. Occupancy sensing controls have been used in conjunction with HVAC systems in the past wherein the HVAC system is either switched off or switched to a higher temperature setting. What is needed is a device and control method for an HVAC system that provides for an energy efficient amount of latent heat removal for use during both occupied and unoccupied periods.
Many attempts have been made to efficiently deal with the dehumidification of enclosed spaces. For instance many prior systems have added separate dehumidification apparatus to conventional HVAC systems and are disclosed in U.S. Pat. Nos. 5,598,715, 5,578,753, 5,427,175, 5,305,822, 5,231,845. U.S. Pat. No. 5,544,809 teaches a method which utilizes algorithm timing strategies to optimize air drying in an enclosed space. This method does not vary the SHR by controlling the air conditioning system component operation. Instead the system in '809 enables the HVAC system at fill capacity for timed intervals and then turns the HVAC system off for timed intervals based on temperature and humidity measurements. The result of such a system is that, as outlined above, the system may take a long time to actually catch up to the latent heat load requirements and high indoor humidity and low outdoor dry bulb conditions. In addition, '809 teaches a method that includes an occupancy sensor that automatically extends the system cycle time if the enclosed space is unoccupied.